There are two principal chemical degradation pathways of paper: acid-catalysed hydrolysis and oxidation. The pH of 1 g of a piece of paper in 50 cm3 of water gives a measure of its acidity. Early work on paper conservation chemistry found that old paper that was alkaline was much stronger than acidic paper.

Paper can become acidic either by absorbing pollutants such as sulfur dioxide and nitrogen oxides, or during manufacture. Traditionally, aluminium sulfate (or 'papermaker's alum') was added to harden, or 'size', the paper. While this gave initial strength, it was also a source of acidity (see Sources of acidity in paper). Most paper made in the century from 1850 is acidic due to manufacturing additives. 

The β-acetal oxygen bridge joining the glucose molecules of cellulose together is susceptible to acid hydrolysis. This breaks the chains and weakens the fibres. Paper that decomposes this way becomes hard and brittle, and disintegrates easily. 

Oxidation, usually initiated by light, discolours the paper. Both cellulose (and its derivatives) and lignin within the paper can be oxidised. In cellulose, oxidising the hydroxyl groups to aldehydes, ketones and carboxylic acids lead to discolouration. But it is lignin that is the main cause of photo-yellowing of paper. It contains several chromophores with conjugated aromatic rings and carbonyl groups that absorb in the near UV spectrum (300–400 nm). When these chromophores absorb light they can decompose into yellow-coloured ketones and quinones, turning the paper yellow. Since these molecules themselves absorb visible light, they act as secondary chromophores and can react further, exacerbating the yellowing and degradation processes.